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Intraspecific variation of organisms is of great importance to correctly carry out taxonomic work, which is a prerequisite for key disciplines in paleontology such as community paleoecology, biostratigraphy, and biogeography. However, intraspecific variation is rarely studied in ectocochleate cephalopods (ammonoids and nautiloids), for which an excessive number of taxa was established during the past centuries. Because intraspecific variation of fossilized organisms suffers from various biases (time averaging and taphonomy), an extant example is needed for actualistic comparison. We applied 3D morphometry to 93 specimens of Nautilus pompilius from three different geographic populations. This data set was used to examine the intraspecific variation throughout ontogeny in detail. Although there are slight differences between the populations as well as some measurement biases, a common pattern of intraspecific variation appears to be present. High variation in morphometric variables appears early in ontogeny and then decreases gradually in the following ontogenetic stages. Subsequently, the variation shows an increase again before maturity until a sharp increase or decrease occurs toward the end of ontogeny. Comparison with intraspecific variation of ammonoids and belemnites illustrated that some groups have ontogenetic patterns of intraspecific variation that are similar to that of N. pompilius. This implies that the abovementioned ontogenetic pattern of intraspecific variation might be common in some major cephalopod clades.

The thickness ratios of shells (=whorl breadth/shell diameter) in the heteromorphic scaphitid ammonoid Scaphites planus (Yabe, 1910) from the lower middle Turonian in the Oyubari and Kotanbetsu areas of Hokkaido, Japan were examined in order to determine their mode of migration. The thickness ratios of S. planus differ significantly between the two localities, which suggest that these different populations did not frequently migrate between the two areas (currently ∼130 km apart, although the actual distance during Turonian is uncertain due to the presence of faults and folds between the two areas). There is no difference in hatchling diameters between the two areas, suggesting that the thickness ratios became manifested after the post-hatchling stage due to limited migration with in a nektobenthic habitat. This study suggests that scaphitid ammonoids became nektobenthic with limited migration at a stage earlier than previously thought (not during the transition from normal to abnormal coiling). The limited migration in scaphitid ammonoids might relate to their higher evolutionary volatility.

The thickness ratios of shell whorls (=whorl breadth/shell diameter) in the ammonoids Damesites sugata (Forbes, 1845), Hypophylloceras subramosum (Shimizu, 1934), and Gaudryceras tenuiliratum Yabe, 1903 from Late Cretaceous outer shelf deposits in the Kotanbetsu area of northwestern Hokkaido, Japan, were examined in order to determine their mode of migration. The thickness ratios of D. sugata differ significantly with depth on the outer shelf environment but show no significant difference laterally at a similar depth. These results suggest that D. sugata did not frequently migrate to different depths on the outer shelf environment but tended to live and migrate laterally at a similar depth on the outer shelf. The thickness ratios of H. subramosum and G. tenuiliratum show no significant difference between different depths on the outer shelf environment or for lateral direction at a similar depth. This suggests that H. subramosum and G. tenuiliratum frequently migrated at different depths on the outer shelf environment and laterally at a similar depth. There is no difference in hatchling diameters in all the examined species at different depths on the outer shelf environment, suggesting that the thickness ratios became manifest after the post-hatchling stage due to limited migration, which would have been in a nektobenthic habitat. The various modes of migration in the study area are reconstructed in this study, suggesting diversified paleoecology of Late Cretaceous ammonoids.

Examination of ontogenetic changes in the septal angle of Late Cretaceous ammonoids (ten species representing seven superfamilies and four suborders) reveals four patterns: 1) a single abrupt change in septal angle; 2) two abrupt changes in septal angle; 3) cyclic fluctuations in septal angle throughout ontogeny; and 4) an almost constant septal angle throughout ontogeny. These various septal-angle patterns in Late Cretaceous ammonoids are in contrast with modern and fossil nautiloids, which have the common pattern displaying a single abrupt change in septal angles. Although the abrupt change of septal angles in nautiloids corresponds with the hatching event from the egg, change of septal angles in the examined ammonoids is hypothesized to correspond not to hatching but to the change from a planktic to a nektobenthic habit demarcated by the post-embryonic stage. Therefore, the variable patterns of septal angles within ammonoids suggest a diverse set of early life histories.

Morphometric analyses of shell morphology in the Cretaceous nautiloid Eutrephoceras clementinum (d'Orbigny, 1840) (Cephalopoda, Mollusca) from the Ariyalur area, southern India, reveal ontogenetic change from hatching to maturity as well as intra-specific variation in shell morphology. the shell breadth has a negative allometric relationship with shell diameter and with whorl height, and the umbilicus diameter has a positive allometric relationship with shell diameter. This shows that shell shape became relatively thinner with less variation, and the umbilicus diameter became relatively broader with growth. the siphuncle position moves from a dorso-central to ventro-central position with growth. A constriction was recognized on the early whorl at 20 mm in shell diameter, and the interval angles of succeeding septa were changed at the 8th septum, indicating that they hatched at this stage. the bending of umbilical walls of apertures toward the center of coiling suggests that E. clementinum attained maturity at about 115 mm in shell diameter. the comparison of the shell morphology of E. clementinum with that of E. bouchardianum (d'Orbigny, 1840) reported in the literature clarifies their difference in whorl shape and umbilical size, especially in the adult stage. This kind of morphometric study of nautiloids is essential for elucidating their adaptive designs for environment and mode of life, functional shell morphology, taxonomy, phylogeny, and evolution.

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